The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides a technique, including a method and apparatus, for improving the quality of films deposited in a substrate processing chamber. The present invention is particularly useful for improving the quality of undoped silicate glass (USG) films deposited by chemical vapor deposition processing, but may also be applied to a number of other types of films, including, merely by way of example, fluorosilicate glass (FSG), phosphorous-doped silicate glass (PSG), boron nitride (BN) and amorphous carbon), as well as to films deposited by other deposition techniques.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition (CVD). Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions can take place to produce the desired film. Plasma CVD processes promote the activation of the reactant gases, typically by the application of radio frequency (RF) energy to excite the reactant gases, thereby creating a plasma. The high reactivity of the activated reactant gases reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such CVD processes. The relatively low temperature of a plasma CVD process makes such processes ideal for the formation of insulating layers over deposited metal layers and for the formation of other insulating layers. A high density plasma CVD (HDP-CVD) process is similar to the plasma CVD process described above, but an HDP process uses inductive energy rather than capacitive energy.
In one prior art HDP-CVD deposition chamber, the vacuum chamber is generally defined by a planar substrate support, acting as a cathode, along the bottom, a planar anode along the top, a relatively short sidewall extending upwardly from-the bottom, and a dielectric sidewall connecting the short sidewall with the top. Inductive coils are mounted about the dielectric sidewall and are connected to a supply radio frequency generator. The top and bottom electrodes are typically coupled to bias radio frequency generators. Two or more sets of equally spaced gas distributors, such as nozzles, are typically mounted to the sidewall and extend into the region above the edge of the substrate support-surface. The gas nozzles for each set are coupled to a common manifold for that set; the manifolds provide the gas nozzles with process gases, including gases such as argon, oxygen, silane (SiH.sub.4), TEOS (tetraethoxy-silane), silicon tetrafluoride (SiF.sub.4), etc., the composition of the gases depending primarily on the type of material to be formed on the substrate. Sets of gas nozzles are commonly used because some gases, such as silane, need to be delivered into the chamber separately from other gases, such as oxygen; other gases, such as oxygen and SiF.sub.4, can be delivered to a common set of nozzles through a common manifold. The nozzle tips have exits, typically orifices, positioned in a circumferential pattern spaced apart above the circumferential periphery of the substrate support and through which the process gases flow.
Possible contamination of the deposition process is always a concern. For example, during deposition of silicon oxide and other layers onto the surface of a substrate, the deposition gases released inside the processing chamber cause unavoidable deposition on areas such as the walls of the processing chamber. Unless removed, this unwanted deposition is a source of contaminates that may interfere with subsequent processing steps and adversely affect wafer yield. To avoid such problems, the inside surface of the chamber is regularly cleaned to remove the unwanted deposition material from the chamber walls and similar areas of the processing chamber. This procedure is performed as a dry clean operation where an etchant gas, such as nitrogen trifluorine (NF.sub.3), is used to remove (etch) the deposited material from the chamber walls and other areas. The etchant gas is introduced into the chamber and a plasma is formed so that the etchant gas reacts with and removes the deposited material from the chamber walls. Such cleaning procedures are commonly performed between deposition steps for every wafer or every n wafers.
However, there are other sources of contamination, such as those contaminates inherent to the deposition chamber itself, that are not eliminated by the cleaning process described above. For example, in the HDP-CVD chamber described above, the dielectric sidewall may be formed of a quartz, silicon oxide (SiO.sub.2) material or of alumina (Al.sub.2 O.sub.3), the nozzles are typically formed of alumina and the chamber body itself is typically formed of aluminum (Al). Each of these materials is a potential source of contamination, including, for example, sodium (Na), lithium (Li), potassium (K), aluminum (Al), iron (Fe), chromium (Cr) and nickel (Ni). Since the contaminates occur naturally within the chamber components, they are generally not eliminated by the above cleaning process and therefore can interfere with processing steps and adversely affect wafer yield.
As an example, it has been observed that sodium is a particularly disruptive contaminant to the deposition process. It is believed that a high level of sodium contamination in the deposition chamber can arise from the relatively high diffusion rate of sodium contaminates or ions through silicon oxide and alumina surfaces. Therefore, it is believed that during wafer processing in a plasma reactor, sodium diffuses through the quartz dome and alumina nozzles until they are free within the chamber, where they become incorporated with and contaminate the deposited film.
A known method of controlling the introduction of contaminates into the deposition chamber involves depositing a "seasoning" layer of silicon oxide over the chamber's interior surface prior to processing substrate films. For example, the clean step described above can, in itself, be a source of particle accumulation. Fluorine from the clean plasma can be absorbed and/or trapped in the chamber walls and can be released during subsequent processing steps. The deposited silicon oxide seasoning layer covers the absorbed fluorine, reducing the likelihood that it will contaminate subsequent processing steps. However, depositing a seasoning layer of silicon oxide does not protect against sodium contamination, for example, because of the relatively high diffusion rate of sodium through silicon oxide to begin with.
Thus, an improved method and apparatus are desired for controlling the introduction of contamination into the deposition chamber that occur naturally within the chamber components.